In general, data stored in a photo diode (PD) of a pixel is transferred to a floating diffusion node which is an input node of a conversion transistor, e.g., a source follower transistor, through a transfer transistor. An output voltage of the conversion transistor, which is output of the pixel and referred to as a pixel signal, depends on the data transferred to the floating diffusion node,
It is desirable to have a linear relationship between the amount the received incident: light at a pixel and the output signal from the pixel. As amount of an incident light on the PD of the pixel is increased, a test for checking a linearity of analog-to-digital conversion is performed and is sometimes referred to as “exposure linearity test.”
The exposure linearity test shows that non-linearity occurs during the operation of an image sensor. For example, it has been observed that when the data of the PD is transferred to the floating diffusion node, non-linearity occurs. The non-linearity is also caused by a pixel structure.
Meanwhile, since a slope of a ramp signal has a linearity, a code error occurs in the pixel signal using the ramp signal.
Recently, the code error is required to be within 1%. However, since the non-linearity of the pixel signal becomes more severe as a size of the pixel signal increases, if the pixel signal has a large size, it becomes difficult to satisfy the required range for the code error and the code error may be beyond the required code error range of 1%.